This disclosure relates generally to internal combustion engines and, more particularly, to engine valve actuators.
Exhaust gas recirculation is a well known concept among internal combustion engine manufacturers. Taking a four stroke diesel engine as an example, one of ordinary skill in the art will readily recognize that such engines operate through four distinct strokes of a piston reciprocating through a cylinder. In an intake stroke, the piston descends through the cylinder while an intake valve is open. The resulting vacuum draws air into the cylinder. In a subsequent compression stroke, the piston reverses direction while the intake valve and an exhaust valve are closed, thereby compressing the air within the cylinder. This is followed by a combustion or power stroke wherein fuel is injected into the compressed air and thereby is ignited, with the resulting force pushing the piston again in the descending direction while both the intake and exhaust valves are closed. Finally, the piston reverses direction with the exhaust valve open, thereby pushing the combustion gases out of the cylinder.
One known disadvantage of such engine operation stems from the byproducts of the combustion process. More specifically, an unacceptably high level of pollutants, such as nitrous oxide (NOx), may be released during the exhaust stroke. Exhaust gas recirculation (hereinafter referred to as xe2x80x9cEGRxe2x80x9d) attempts to curtail such drawbacks of conventional engine operation. With EGR, at least a portion of the exhaust gases, and thus a portion of the combustion byproducts, is not exhausted to atmosphere, but rather is introduced back into the engine cylinder to be combusted in subsequent power or combustion strokes of the engine.
EGR can be performed internally or externally. With external EGR, a conduit or other form of pathway is provided to direct the exhaust gases expelled through the exhaust valve back to the intake valve. While effective, such an approach requires additional engine components, including the aforementioned conduit, thereby increasing weight and size requirements and decreasing efficiency.
With internal EGR, such conduits and additional engine components are avoided. Rather, the exhaust gases expelled through the exhaust valve are re-introduced to the cylinder through the exhaust valve itself. Such a process requires the exhaust valve to stay open not only through the exhaust stroke, but also after the piston reverses direction, thereby creating a vacuum and drawing a portion of the exhaust gases back into the cylinder through the still open exhaust valve.
One of ordinary skill in the art will readily appreciate that the force required to open the exhaust valve, and maintain the exhaust valve in an open position as the piston reciprocates through the cylinder to a top dead center location, is substantial. Conventionally, the exhaust valve has been held in such an open position by a valve actuator employing highly pressurized oil. More specifically, a valve actuator, having a cylinder in which a piston is reciprocatingly disposed, is provided proximate a stem of the exhaust valve. Movement of the actuator piston, by the highly pressurized oil, to an extended position imparts opening force to the valve stem.
In order to allow for internal EGR, pressurized oil on the order of, for example, fifteen hundred to five thousand pounds per square inch (10.34 to 34.4 MPa) has had to be supplied to the valve actuator (other pressure ranges are possible). The engine or machine in which the engine has been mounted therefore has had to provide a high pressure source or rail and be able to supply the high pressure oil to the actuator when EGR is desired. Such a requirement has, among other things, the disadvantage of decreasing the engine efficiency in that the engine must continually direct substantial usable work to the high pressure rail to maintain such pressures even though the high pressure oil is only required for a relatively short duration during engine operation.
The present disclosure is directed to overcoming one or more of the problems or disadvantages associated with the prior art.
In accordance with one aspect of the disclosure, an engine valve assembly is provided which comprises an engine valve seat, an engine valve element adapted to move relative to the engine valve seat between an open position and a closed position, and an actuator operatively associated with the valve element and adapted to impart a first force having a first magnitude to the valve element, and a second force having a second magnitude greater than the first magnitude to the valve element to move the valve element.
In accordance with another aspect of the disclosure, an engine valve actuator is provided which comprises an actuator cylinder, an actuator piston reciprocatingly mounted in the actuator cylinder, the piston including a rod extending therefrom, a first source of pressurized fluid, a second source of pressurized fluid, the second source being pressurized at a higher pressure than the first source, and a control valve in fluid communication with the first source, the second source, and the actuator cylinder. The control valve has a first position and a second position, with the first position connecting the first source in fluid communication with the actuator cylinder, and the second position connecting the second source in fluid communication with the actuator cylinder.
In accordance with another aspect of the disclosure, an engine is provided which comprises an engine cylinder, an engine piston reciprocatingly disposed in the engine cylinder, a valve element disposed in a port of the engine cylinder, a valve actuator connected to the engine and positioned proximate the valve stem, a first source of pressurized fluid in fluid communication with the valve actuator, a second source of pressurized fluid in fluid communication with the valve actuator, and a control valve adapted to control flow of pressurized fluid from the first and second sources of pressurized fluid to the valve actuator.
In accordance with another aspect of the disclosure, a method of controlling an engine is provided which comprises providing an engine having an engine cylinder, a valve port, and a valve adapted to open and close the valve port, opening the valve with a first force derived from a first source, holding the valve open with a second force derived from a second source, expelling exhaust gas from the engine cylinder during the opening step, and drawing exhaust gas into the engine cylinder during the holding step.